1. Field of the Invention
The present invention relates to an oxygen sensor probe for a boiler, and in particular to an oxygen sensor probe suitable for measuring oxygen in the exhaust gas being discharged from a heating engine such a small boiler.
2. Description of the Prior Art
Recently, it has become considerably important to control harmful gases and floating particles included in exhaust gas being discharged from various heating engines as well as boilers and automobile engines in view of the fact that atmospheric air pollution has become a very serious problem.
An oxygen sensor, especially, is used for measuring the amount of oxygen in the exhaust gas in order to control and maintain a ratio of air to fuel and to control and maintain the operation of heating engines such as a boiler at an optimum state thereof, and for controlling the amount of oxygen supplied at an appropriate level.
At present, most oxygen sensors for a boiler use stabilized zirconia as an element of the oxygen sensor.
As shown in FIG. 3, a conventional zirconia oxygen sensor has a U-shaped element of zirconia inserted into a probe shell made of strong anti-corrosive stainless steel. It comprises a zirconia oxygen sensor element 31 of U-shape, a heater 32, a plug 33, and a probe shell 34.
The convex closed portion 31' of the zirconia element 31 is arranged so as to contact with the exhaust gas to be measured, and a thermocouple 36 is inserted into the concave open portion 31" from its opposite end, thereby giving an exposure to the air as a reference gas.
The lower end 37 of the U-shaped zirconia element 31 is attached to the inside wall of the plug 33 which is also made of a strong anti-corrosive stainless steel. The heater 32 is installed in the inside wall of the probe shell 34, and the upper portion of the probe shell 34 in contact with the exhaust gas is covered with a protective filter 35.
Accordingly, the space between the zirconia element and the probe shell is completely cut off from the outside by the protective filter and plug so that the exhaust gas and air are completely isolated from one another in the sensor.
FIG. 4 shows the structure of another conventional oxygen sensor. The oxygen sensor has separate heater tubes 32' on the inside of the probe shell 34, instead of the heater being located on the inside wall of the probe shell.
As shown in FIGS. 3 and 4, the conventional oxygen sensor is an insertion-type sensor, in which the element is inserted into the probe shell made of stainless steel. Hence, it has the disadvantage of not being useful for boilers of 5 tons or less because of its large scale.
Moreover, the zirconia element of the conventional oxygen sensor needs to be long in order to provide room for the heater as shown in FIG. 3, and the outer diameter of the probe shell needs to be large to accommodate the heater tubes as shown in FIG. 4.
While measuring the amount of oxygen in the exhaust gas by using the oxygen sensor, the exhaust gas should not be in contact with air. Therefore, it is very important to provide an adequate seal between the lower end of the zirconia element and the plug to completely isolate the exhaust gas from the air in the sensor.
Since the exhaust gas contains a lot of various corrosive gases, the material of the plug and the probe shell have generally been of a strong anti-corrosive stainless steel. However, the thermal expansion coefficient of stainless steel is very large, as much as 19.times.10.sup.-6 so that it is difficult to bind the stainless steel with the zirconia element, which has a thermal expansion coefficient of about 9.times.10.sup.-6.
The reason is explained in detail as follows. The zirconia element and the plug made of stainless steel are bound in a circumferential direction. In this case, there occurs an inevitable difference of thermal expansion coefficient between them of up to 10.times.10.sup.-6. If the temperature of the zirconia element and stainless steel is 800.degree. C. or more, a difference of distortion to an extent of 8.times.10.sup.-3 (derived from a difference of thermal expansion coefficient) may occur. No materials can resist such distortion. Further, since the zirconia element is cylindrical in shape, more stress is concentrated in the circumferential direction than the vertical direction, and a shear stress occurs between the attached portion of the element and its portion exposed to the air. As a result, a fracture may occur.
In order to solve this problem, an attempt has been made to alter the zirconia element from a cylinder to a disk. A portion to be exposed to the air is removed, and the outside metal cylinder is thin enough to be easily distorted. However, the method is not desirable because the binding portion is effected by heating the binding portion at a temperature of 700.degree. C. or more, and there is a problem in connecting the electrode because of high temperature.
Stainless steel can be used even under an inferior condition due to its beneficial properties of being strongly resistant to chemical agents and corrosion. Accordingly, an attachment of ceramic to stainless steel has been attempted since its range of application will be enlarged if the attachment with ceramic is possible. Ceramic may be used where only the strength of the attachment is important, but not where a high level of air tightness is required. However, the technology for an air tight attachment between a U-shaped zirconia element and stainless steel has not yet been developed.
In particular, in case of a zirconia oxygen sensor for a boiler, in which the zirconia element is heated by using a heater, the overall size and shape of the sensor probe will depend on the sealing method used.
The following sealing methods have been reported:
1) extending the length of the U-shaped zirconia element and measuring the amount of oxygen at a discharging portion of the exhaust gas, so that the binding portion is exposed to the atmosphere to prevent thermal shock.
2) using a talc powder between the U-shaped zirconia element and the plug instead of attaching the zirconia element directly to the plug; and
3) sealing a disk-type zirconia element to the plug in the form of a tube by using materials such as glass or a brazing alloy.
However, in the method of 1), the size of the sensor becomes too large due to the extended length of the zirconia element, and it is difficult to select a sealing material suitable for the temperature of the sealing portion. Furthermore, the sealing function of the sealing material can be a problem if there is a difference in pressure between the inside of the boiler and the atmosphere of over 500 mm H.sub.2 O.
The method of 2) has the feature that the sealing portion can resist a high temperature of 500.degree. C. or more and the size of the sensor is miniaturized to a certain extent. On the other hand, the size and thickness of the outer plug needs to be enlarged to obtain an air-tight seal that can enclose the strong pressure deriving from the filling of the talc powder with strong pressure. It results in an increase in the size and weight of the sensor. If a difference in the pressure between the atmosphere and the inside of the boiler is over 500 mm H.sub.2 O, the sensor is hard to use because the airtight sealing is not equal to the attached version.
The method of 3) in which a disk-type zirconia element is attached to a tube-type plug has no problem of sealing. However, this method has the disadvantage that the life of the sealing portion is shortened when heated to a high temperature of over 700.degree. C. since both the zirconia element and the sealing portion are heated by a heater, and the sensor deteriorates when heated below this temperature. Since the electrode is connected by mechanical pressing at the above temperature, the connection of the electrode becomes unstable. Moreover, the miniaturization of the disk element is limited because the strength and size of the disk are given much weight. Consequently, the operation becomes inconvenient because of the increased weight of the probe, an error in electromotive force occurs due to the unstable electrode, and the life of the product is shortened.
In particular, in the case of using a glass or brazing alloy, a compressive stress is located on the portion of the zirconia element, thereby giving a shear stress between its sealed outer and inner surfaces to be exposed to the atmosphere. Consequently, a cracking and/or rupture may occur in the sealed portion.
On the other hand, the durability of the oxygen sensor probe is very important. If the durability of the oxygen sensor probe increases two times, it is equivalent to two oxygen sensor probes. The most problems with durability are a high corrosion (resulting from reacting platinum with SO.sub.2 in the exhaust gas in the vicinity of temperature of 300.degree. C. to 400.degree. C.) and a low corrosion (resulting from reacting platinum with H.sub.2 SO.sub.4 in the exhaust gas in the vicinity of temperatures of 150.degree. C. to 300.degree. C.). Since SO.sub.2 or H.sub.2 SO.sub.4 in the exhaust gas will react with the platinum, the platinum electrode will deteriorate. To prevent this corrosion, a plasma jet coating with spinel can be used.
However, the contact of platinum and exhaust gas cannot be completely prevented since spinel is porous. Further, the reaction rate in the sensing portion will drop on account of the above coating.